Traditionally, such profiled tubes are made by molding them in relatively complex multi-part molds or, for example for flexible profiled tubes, by using a former mandrel with machined hollows or grooves with the tube pressed into the bottoms of the grooves of the former with a cable or other pressure applying device. These tools are expensive and the steps of the shaping process are relatively time consuming and complex.
The present invention provides a relatively rapid process, which uses relatively inexpensive tools, and which allows the manufacture of a wide range of shaped tubes of very different thicknesses and diameters, and in which the profiled tubes made have very good thermal stability.
The process according to the invention is essentially characterized by the fact that a tube made of a plastic material which has memory return characteristics when heated, is heated to a temperature slightly above the softening point, is placed in a mold whose internal shape essentially duplicates the desired final external shape for the tube, is subjected while hot and in the mold to an internal pressure to expand and press or plate the tube against the internal wall of the mold, so as to give it the profile shape of the mold, and the mold is cooled while internal pressure is maintained in the tube. The cooled tube is then pulled from the mold, but the profiled portions of the tube distort during such removal. The tube is then again heated to a temperature below the softening point, to return the tube, by virtue of its plastic memory, to the shape it had while in the mold.
The first heating step can be done while the tube is held in the mold, and is preferably carried out in a heated enclosure for a sufficient period of time to increase the temperature of the tube to above the softening point of the plastic material.
In practice, for thin tubes made of PTFE for which the softening point is 327.degree. C. and for the usual range of profiled tube thicknesses between 0.1 and 6 mm, and diameters between 2 and 300 mm, the heating is carried out in an enclosure brought to a temperature slightly higher than the softening point, specifically around 350.degree. C.
The cooling step during which the formation under pressure takes place is advantageously carried out over a period ranging from about 2 minutes to about 40 minutes, the final temperature reached by the tube before being removed from the mold being about 20.degree. C.
The second heating phase which returns the tube to the shape acquired while in the mold, by plastic memory, is very short, and for tubes of the dimensions listed above, the heating time is between 5 seconds and about 10 minutes at a temperature about 10% lower than the softening temperature, particularly about 310.degree. C., where the plastic material is PTFE.
If desired, the resultant shaped tubes can be reworked after cutting, to provide cylindrical ends by localized heating of the ends above the softening point, and then reforming the ends.
The resultant tubes have dimensional stability up to the softening temperature of the plastic material. In addition, the tubes retain such stability in the usual ranges of use for the resultant tubes, for example, for PTFE from -30.degree. C. to +260.degree. C.
The present invention also relates to apparatus for implementing the described method, this apparatus including a non-split unitary mold body comprising an elongated tubular body, whose internal wall has a shape similar to that which is desired for the tube and two end flanges each of which can be connected to an end assembly including plugs for holding the ends of the tube to be shaped and for sealing the interior of the tube when it is in the mold. At least one of the plugs has a port connectable to means for pressurizing the inside of the tube held in the mold.
The tubular body of the mold can advantagously be a simple, relatively thin wall metallic sheath, particularly of stainless steel, having a corrugated shape, in the form for example, of helical or parallel corrugations, the mold thus having a very low thermal mass allowing very short heating and cooling times.
Preferably, each end assembly has a casing unit including means for connection to the corresponding flange of the mold, and an internal, preferably frustoconical interior in which plugs of corresponding shape engage to hold the end of the tube between the outside of the plug and the inside of the casing unit, at least one of the plugs having a duct or port which can be connected to a source of fluid under pressure.
To implement the process decribed above using the apparatus according to the invention, a smooth tube slightly longer than the mold is placed in a mold of the type described, the tube is secured in the mold with the end assemblies, the assembly is placed in a heating enclosure for a period of time sufficient for the temperature of the tube to go beyond the softening point of the plastic material, the assembly is withdrawn from the heating enclosure while internal pressure is maintained in the tube through at least one of the plugs during cooling, then one of the end assemblies is removed and the tube is extracted from the mold by longitudinal traction by means of the other end assembly. Then, after removing the tube from the end assembly, the tube is placed in a heating enclosure to return it by plastic memory to the shape of the mold, and which was lost as a result of elongation during the step of extraction from the mold.
To better understand the invention, reference is made to the following description, as a non-limiting example of a preferred embodiment of the invention, with reference to the accompanying drawings.